We have used a scanning tunneling microscope (STM) to determine a site conversion path of a Ti atom on the Si(001)-2×1 surface and the activation energies. We have found that adsorption of Ti between 442 K and 488 K is described by two precursor states and a final state. A Ti adatom is converted reversibly between the two precursor states, i.e., the pedestal site adsorption and a highly mobile state between the dimer rows. In contrast, the final state, i.e., the vacancy site adsorption is created only from the pedestal site adsorption. By counting individual events of two types of the site conversions, i.e., one from the pedestal site adsorption to the vacancy site ( type-A ) and another to the highly mobile state ( type-B ), we have obtained activation energies of ε_A = 1.6 ±0.1 eV and ε_B = 1.6± 0.2 eV for type-A and type-B, respectively. The conversion paths and the deduced activation energies are consistently interpreted in terms of the local bonding nature of a Ti adatom on the Si dimer reconstruction.

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